fetal DNA screening) available in the United States. Blood testing is also used to look for abnormal levels of alphafetoprotein or hormones. The results of all three factors may indicate a higher chance of Down Syndrome. If this is the case, the woman may be advised to have a more reliable screen such as cell-free fetal DNA screening or an invasive diagnostic test (such as chorionic villus sampling or amniocentesis). Screening for Down syndrome by a combination of maternal age and thickness of nuchal translucency in the fetus at 11–14 weeks of gestation was introduced in the 1990s. This method identifies about 75% of affected fetuses while screening about 5% of pregnancies. Natural fetal loss after positive diagnosis at 12 weeks is about 30%. Other chromosomal defects Other common chromosomal defects that cause a thicker nuchal translucency are Turner syndrome Trisomy 18 Trisomy 13 Triploidy Other defects with normal karyotype In fetuses with a normal number of chromosomes, a thicker nuchal translucency is associated with other fetal defects and genetic syndromes. Procedure Nuchal scan (NT procedure) is performed between 11 and 14 weeks of gestation, because the accuracy is best in this period. The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head (neither hyperflexed nor extended, either of which can influence the nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from leading edge to leading edge. It is important to distinguish the nuchal lucency from the underlying amniotic membrane. Normal thickness depends on the crown-rump length (CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality. Accuracy An increased nuchal translucency increases the probability that the fetus will be affected by a chromosomal abnormality, congenital cardiac defects, or intrauterine fetal demise. Typically, nuchal translucency alone is not sufficient as a screening test for chromosomal abnormalities. How to define a normal or abnormal nuchal translucency measurement can be difficult. The use of a single millimeter cutoff (such as 2.5 or 3.0 mm) is inappropriate because nuchal translucency measurements normally increases with gestational age (by approximately 15% to 20% per gestational week from 10 to 13 weeks). At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed; however, up to 13% of chromosomally